peerflix is the same software that the online service popcorn-time uses in the background. You can also stream and watch torrent or magnet links on your laptop or desktop locally using peerflix.
To install peerflix on Ubuntu:
$ sudo apt-get install nodejs $ sudo npm install -g peerflix
To stream a torrent and watch using smplayer (you can use any media player that understands streams):
peerflix "magnet:?xt=urn:btih:ef330b39f4801d25b4245212e75a38634bfc856e" --smplayer
The torrent or magnet URL must be within double quotes as they contain all kinds of characters.
When started peerflix will show a local address (and port) which can be passed to the media player independently to start the streaming. If the torrent has multiple files in it, perflix will choose the biggest one for streaming.